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Re: [Xen-devel] questions regarding interdomain communication and xen-guest communication

>>> Aastha Mehta <aasthakm@xxxxxxxxx> 10/24/13 7:32 PM >>>
>On 24 October 2013 09:09, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>>> Aastha Mehta <aasthakm@xxxxxxxxx> 10/23/13 7:44 PM >>>
>>> In case of an asynchronous I/O does one guest have to
>>>register its callbacks with the other guest directly?
>>>Or does one guest hypercall into xen, which then upcalls into the
>>>other guest and then transfer reply in the reverse using another pair
>>>of hypercall and upcall?
>> Again, the latter.
>So when one guest (G1) shares pages with second (G2), for example for
>block I/O, G1 will inform second via xen about a shared page.

How this information is propagated is up to you; commonly this is being
done via xenstore.

> G1 will also register a callback with Xen.

That callback will need to be registered in any case - the (general)
event upcall.

> When Xen upcalls into G2 to forward G1's request, does it block on G2's 
> reponse?

Why would it - it acts as a passive agent here only.

> How does G2 send back the result of I/O to G1?

Generally as contents of the shared page.

But really - please take a look at the various existing backend/frontend

>>>Regarding xen -- domain communication: how can xen send some request
>>>to dom0 for some operation that needs to be performed in the dom0 and
>>>get back results from the dom0?
>> Look at the MCE polling Dom0 performs for example, or the memory pressure
>> threshold handling: One model would be for Dom0 to set up a (dedicated)
>> vIRQ for that purpose, and Xen trigger that vIRQ when it needs Dom0's help.
>Just to clarify, will guest return the result of the operation using a

First of all there's generally no "result" in the examples I gave. Instead,
Xen simply requests a certain kind of action to be taken. And Xen in fact
must not depend on whether or when that action is being taken. But in
general it might well be that the action requested consists of one or more


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